Magnetic recording media for high density recording have been developed, and thus better surface smoothness of the magnetic layer of the magnetic recording medium is required to reduce space loss between a magnetic head and a magnetic recording tape. For this purpose, it is necessary to improve the surface smoothness of a magnetic layer. The surface smoothness of a magnetic layer can be improved by improving methods for preparing magnetic recording media, for example, with respect to method of dispersing the magnetic particles, coating methods, surface molding methods (e.g., calendering treatment) and by improving surface smoothness of a support thereunder. Also, attempts have been made to make magnetic recording layers thinner in order to reduce loss which is caused due to the thickness of the layer, because the recording wavelength becomes shorter with increases in the recording density. Accordingly, the higher surface smoothness of the support is required as the thinner magnetic layer is provided thereon.
However, it is limited by the following reasons to increase the surface smoothness of a plastic film support which is used for a magnetic recording medium. That is, if the surface smoothness of the support is good, it does not move smoothly in the step of winding it which is conducted after a film-molding (e.g., calendering) step, and wrinkles generate on the film, because the friction resistance of the film to rollers for transporting the film is increased. Also, the shape of a wound film is not uniform, because friction resistance between opposite surfaces of the rolled film is increased.
Various methods have been proposed to overcome the foregoing problems. For example, it is disclosed in Japanese Patent Application (OPI) No. 109,605/83 (the term "OPI" as used herein means a "published unexamined Japanese patent application) that fine particles of thermoplastic resins which are present on a support can be removed by dissolving them with a solvent, and then a magnetic layer is coated on the support. However, even this method has not been able to provide satisfactory characteristics necessary for a magnetic recording medium used for high density recording medium.